1. Field
This invention relates to shunt calibration systems for transducers and, more particularly, to improvements in such systems in which rotary transformers are used to couple excitation and signal to and from a transducer mounted on a rotating member.
2. Prior Art
Rotating shaft torque transducers often utilize the maintenance advantages of rotary transformers for signal transfer. Rotating transformers differ from conventional transformers only in that either the primary or secondary winding is rotating. Such rotary transformers are generally described in U.S. Pat. No. 3,611,230 entitled "Rotary Transformer Structure". In applications where a strain gage transducer employing a Wheatstone bridge circuit is placed on a rotating member, a first rotary transformer is used only to transmit an AC bridge supply voltage to the bridge, while a second transformer is used to pick off the output voltage. Thus, through the use of the rotating transformers, slip rings are eliminated and there is no direct contact between the rotating and stationary elements of the sensor. Although this isolation is advantageous, it requires an external or dummy bridge, or a ground reference network, as disclosed in U.S. Pat. No. 3,790,811 to be used on the instrument side of the transformers. This circuitry provides a current return path for the AC carrier instruments generally used in transducer systems to properly monitor the output of the transducer.
A more complete understanding of rotary transformer systems as used in conjunction with transducers may be gained with the aid of FIGS. 1 and 2. Referring first to FIG. 2, a generalized transducer system is shown in which a carrier or source of alternating current 206 is coupled to the primary winding 208 of the first transformer 201. For ease of reference, this is referred to as the system excitation. The secondary winding 210 of the second transformer 204 couples the output voltage to an output or utilization device 207. This is referred to as the system signal. It must be appreciated at this time that a single "instrument" can provide the function of a carrier and a utilization device and in fact such instrument may be calibrated to provide an output reading of the property to be measured by the transducer system.
Referring to FIG. 1, a transducer system is illustrated for measuring the torque on a rotatable shaft 101. The excitation supply to the input of the system is coupled to the transducer on shaft 101 by a first transformer 102. Transformer 102 includes a stationary or primary winding receiving power from a connecting block 104 and a rotatable or secondary winding 105 connected to a strain gage transducer 106. Also shown is a second transformer 107 having a rotatable primary winding 110 coupled to the transducer and a stationary secondary winding 108 transmitting an output signal to the connecting block 104. Both the transformers and the transducer are contained within a housing 109.
When a rotary transformer system is utilized in conjunction with a strain gage bridge to measure the torque on the shaft, it is desirable to calibrate the bridge relative to the electrical instrumentation. Such calibration is necessary because of normal variations in the electrical equipment which provides the multiple functions of supplying the excitation to the bridge and which receives the output signal from the bridge. In addition, it is sometimes desirable to calibrate the equipment while the shaft is rotating.
To calibrate a strain gage bridge, a well known technique is to apply a resistor across one leg of the bridge. This technique is called shunt calibration because the resistor is connected in shunt or parallel across the bridge.
Prior to the present invention, calibration of the transducer in a rotary transformer system was very complex. The dummy bridge permitted a convenient method of performing a shunt calibration that is otherwise not possible on the shaft. This method of shunt calibration which is external to or removed from the rotating transducer, is in general, valid; however, it has certain limitations which restrict its use to controlled conditions which are difficult to obtain in actual practice.
Input impedance unbalance, found in some instruments, necessitates the use of a resistor-capacitor correction network with the dummy bridge. With an external shunt calibration system, the simulated shunt phase and the actual signal phase are frequency dependent and usually will not match. The shunt signal must be shifted to be in phase with the actual signal at the carrier drive frequency. This is accomplished by adjusting the values of the R-C network. The frequency sensitivity of this system limits its usefulness.
A prior art technique designed to overcome the disadvantages of the dummy bridge method of calibration is described in U.S. Pat. No. 4,059,005. In this technique, use is made of a magnetically actuated switch, mounted on the rotating shaft which is activated by introducing a magnetic field of sufficient strength. The switch, which is placed in series with a calibration resistor to connect it in parallel with an appropriate leg of the bridge, can be activated and used in static and dynamic conditions, regardless of shaft position by utilizing magnetically conductive rings.
The magnetically actuated switch technique uses many of the usual circuit components associated with measurements on rotating equipment. The circuitry of this technique will be dealt with in detail here because it provides a foundation for the present invention and it clearly shows the advantages of the present invention in eliminating the complication and cost of this prior art approach.
FIG. 3 illustrates a rotary transformer transducer system which is arranged to measure the torque of a rotatable shaft 308 and incorporates a magnetic switching system for calibration. The input signal is coupled to the shaft by a transformer 201 having a stationary winding 302, connected to block 301, and a rotatable winding 303 connected to a strain gage transducer 304 which incorporates a bridge. The output is taken from the bridge by a second transformer 204 having a rotatable winding 306 connected to the transducer and a stationary winding 305 connected to the connector block 301.
A schematic representation of this system is shown in FIG. 5 with the components on the left of the dash line being mounted on the rotating shaft 308 and the components to the right of the dash line being mounted in the stationary housing 307. Specifically, stationary winding 302 and 305, provide the system input and output respectively. These windings are secured within the housing 307 on the stationary side, while rotating windings 303 and 306 and the strain gage transducer 304 are all mounted on and rotate with the shaft 308. The strain gage transducer 304 is a bridge circuit which includes four legs 501 through 504.
The above described circuitry is common to most rotational torque measurement systems, however, it is the magnetically responsive switch that distinguishes this technique from other prior art approaches. In this embodiment, a shunt impedance 319, preferably a resistor, is selectively connectable across a first part of the strain gage bridge such as across leg 501 from a first terminal 505, defined as the junction of legs 501 and 504, through a magnetically responsive switch, such as a reed switch 309, and then to the common connection between legs 501 and 502 of the bridge. Upon closing switch 309 the impedance 319 is connected across leg 501. In order to actuate reed switch 309, an electromagnet 512 is provided having pole faces 511 and 516. Energization of magnet 512 closes switch 309 to couple the impedance 319 across leg 501, while de-energization of the magnet permits the reed switch contacts to open. Thus, reed switch 309 is a normally open type of hermetically sealed reed switch.
If a single reed switch 309 is secured to the rotatable shaft 308, when the reed switch is 180.degree. away from the pole faces of the electromagnet, a stronger magnetic flux density is required to close the contacts of the reed switch than when switch 309 is adjacent the pole faces. Thus, the switch is position-sensitive, i.e., its distance away from the magnets may cause it to be free of influence from the magnetic field.
To eliminate position sensitivity and to minimize the effect of other variables such as shaft speed, shaft diameter, reed switch characteristics and magnet configuration, this prior art approaches uses three reed switches, 309, 309A and 309B, as illustrated in FIG. 4, each of which is electrically connected in parallel and supported by a ring assembly 310 and 312. The pole faces 511 and 516 of the magnet are positioned to oppose the steel rings. Thus, upon energization of the magnet 512, the steel rings concentrate the magnetic flux to assure that at least one reed switch closes.
The foregoing provides single shunt calibration i.e., positive or negative. However, if both positive and negative shunt calibration are desired, then the structure must include all the elements of FIG. 4. Specifically, ring 311 and switches 313, 313A and 313B. A second electromagnet 513 having pole faces 514 and 515 is provided as in FIG. 5 to actuate the second series of reed switches. This places the resistor 319 across leg 504 of the bridge.
The positive and negative shunt calibration method described above is preferred over the dummy bridge method because in the dummy bridge method both transformers are required to be perfect or their deficiencies must be compensated for this method to provide accurate results.
Although the reed switching method does permit direct application of the shunt calibration resistor to a transducer, the complication in realizing this advantage is quite significant. Multiple switches and magnetic rings are required. In addition, it has been found that on accelerating of the shaft to which the reed switches are mounted, unintentional closure of the switches occurs, resulting in erratic measurement results.
Slip rings are, in principle, simple devices that have long been used to make contact with rotating equipment. The main problem associated with slip rings is the continuous fricional wear. In addition, where power is applied, arcing occurs producing pitting which tends to produce a roughened surface, further increasing the fricional wear. These and other problems associated with slip rings have made the rotary transformer a preferred choice for instrument measurements. Unfortunately, the rotary transformer is costly and not always necessary, especially where a rotary measurement is not necessary or where it may only be required intermittently. For example, there is often no need to calibrate a transducer while the transducer is rotating. In such cases, systems employing rotary transformers for shunt calibration are excessively costly.